1. Technical Field
The disclosure relates to pneumatic tire inspection and more particularly to a system and method for detecting tire zippers.
2. Description of the Problem
Pneumatic tire defects present serious cost and safety problems, particularly on the retreaded tires commonly used with over the road trucks. Perhaps the most damaging defect is the “zipper”, a breakage of the reinforcing cords that can span the tire radially from bead to bead. Zippers are usually the result of driving on an under-inflated tire which in turn results in cord-damaging flexure and overheating of the sidewall. The term “zipper” refers both to the appearance of the broken cords and to the pattern of their spread. Since each cord depends on its neighbors for support, the failure of one cord makes the failure of an adjacent cord, and each subsequent cord, more likely.
The inspection of tires for zippers has frequently been done by human technicians. A great deal of effort has gone into developing automated techniques for the detection of zippers and other tire-defects in order to standardize and expedite the process. Among the automated inspection techniques are systems based on X-ray, acoustical, electrical, and mechanical scans. Tires are subjected to a number of pressure regimens, such as vacuum, standard air, single, and dual pressures. All of these techniques are of some value in discovering defects, yet they haven't displaced inspections by trained human inspectors, particularly for detecting zippers. In an industry interested in safety improvements and economy of operation, consistent and predictable methods amenable to widespread use for zipper detection, even as a complement to human inspection, are desirable.
Automated inspection regimens based on dual-pressure approaches have achieved the most success. Dual pressure systems involve mounting a tire for rotation relative to sensors and inspecting the tire at different levels of inflation. Typically, the tire is first inflated to a baseline pressure and the external surface is scanned while the tire is rotated. Then the tire is inflated to a second, higher pressure and examined again while rotating to determine if the tire's surface exhibits differential degrees of expansion under the higher pressure. Significantly defective and hence weaker areas will exhibit differential displacement relative to the rest of the tire as the pressure change. While it is to be expected that the two height profiles will differ, scans of a tire with weaker areas will result in height profiles that vary unevenly. It is also possible to start with a higher inflation pressure as the base line and then reduce pressure in the tire and look for differential degrees of contraction.
U.S. Pat. No. 6,616,650 granted to Mähner may be taken as representative of the art relating to the dual pressure methodology for examination of a tire. Light sections are projected onto the exterior surfaces of the sidewalls of a tire, and the tire is rotated relative to a camera. The camera captures the reflection of the projected light sections at defined rotary positions of the tire relative to the camera and data processing equipment determines a data representation for the shapes for the captured section images. Differential changes in shape as a byproduct of changes in pressure are then isolated as indicators of possible defects.
A common feature in the prior art is provision for having the tire rotate relative to sensors. This may involve having either the tire rotate or the sensors revolve around the tire's usual axis of rotation.